This invention relates to a medium and process for generating acid using a sensitizing dye and a supersensitizer. More specifically, this invention relates to such a medium and process which improves the radiation sensitivity of cationic electron acceptors.
It has long been known that certain cationic electron acceptors (for example, certain phosphonium, sulfonium, diazonium and iodonium salts, and certain pyridinium and pyrylium compounds) will generate powerful acids when exposed to electromagnetic radiation of an appropriate wavelength. Most such cationic electron acceptors are themselves only sensitive to far ultra-violet radiation (below about 300 nm), but they can be used in conjunction with a sensitizing dye which sensitizes the cationic electron acceptor to longer wavelength radiation in the near ultra-violet, visible or even infra-red ranges. The acid generated from the cationic electron acceptors can be used for a variety of purposes, for example polymerization, depolymerization, or causing a color change in an indicator dye; imagewise exposure of a medium containing an cationic electron acceptor and such an indicator dye may be used to form an image.
For example, U.S. Pat. Nos. 5,286,612; 5,453,345 and 5,578,424 describe processes for generation of acid from a cationic electron acceptor (usually an iodonium salt) using an imagewise infra-red exposure in the presence of a sensitizing dye followed by a blanket ultra-violet exposure. In a preferred process, the acid generated from the cationic electron acceptor (this acid is referred to in these patents as a "superacid" and may hereinafter be referred to as the "primary acid") is heated while admixed with a secondary acid generator, a compound capable of undergoing thermal decomposition to produce a secondary acid, the thermal decomposition of the secondary acid generator being catalyzed by the primary acid. The heating of the primary acid/secondary acid generator mixture causes "amplification" of the primary acid, that is to say each mole of primary acid present as a result of the infra-red and ultra-violet exposures produces multiple moles of secondary acid. The preferred secondary acid generators described in this patent are squaric acid derivatives and oxalates, and the secondary acid may be used to change the color of an indicator dye.
U.S. Pat. No. 5,401,607 describes a process for generating acid by exposing a mixture of a diaryliodonium salt and a squarylium dye to infra-red radiation.
U.S. Pat. Nos. 5,422,230 and 5,451,478 describe a process in which the process of the aforementioned U.S. Pat. No. 5,286,612 is used to create a slide which does not require chemical development.
U.S. Pat. Nos. 5,441,850 and 5,631,118 describe processes generally similar to that of the aforementioned U.S. Pat. No. 5,286,612, but in which the imagewise exposure can be in the visible region. In these patents, the sensitizing dye has a first form which absorbs strongly at the wavelength used for the exposure and thus sensitizes the cationic electron acceptor to this wavelength, and a second form which has much lower absorption at this wavelength. The latter patent mentions that it is desirable that the layer or phase containing the sensitizing dye and the cationic electron acceptor also comprise a supersensitizer (called a "cosensitizer" in this patent), which is a reducing agent less basic than the secondary acid generator; the presence of such a supersensitizer greatly improves the quantum efficiency of the reaction between the photoexcited sensitizing dye and the cationic electron acceptor, and thus the sensitivity of the imaging medium. Preferred supersensitizers include triarylamines (for example, triphenylamine) and hydroquinones.
U.S. Pat. No. 5,582,956 describes processes which are generally similar to those of the aforementioned U.S. Pat. Nos. 5,441,850; 5,631,118 and 5,286,612, but in which, following the color change in the indicator, there is reacted with the cationic electron acceptor a reactive material which irreversibly destroys this acceptor, thus rendering the imaging medium insensitive to radiation and fixing the image.
Application Ser. No. 08/757,195, filed Nov. 27, 1996, describes processes which are generally similar to those of the aforementioned U.S. Pat. Nos. 5,441,850; 5,631,118 and 5,286,612, but in which the secondary acid generator has a first site bearing a first leaving group and a second site bearing a second leaving group, the first leaving group being capable of protonation by the primary acid produced by the cationic electron acceptor, with expulsion of the first leaving group, followed by loss of a proton from the secondary acid generator to form an unstable intermediate, which then fragments with loss of the second leaving group, accompanied by either (a) loss of a second proton; or (b) addition of a proton-containing nucleophile, followed by loss of a proton, the second leaving group, in combination with a proton, forming the secondary acid. Preferably, the secondary acid is capable of protonating the first leaving group of the secondary acid generator, so that the thermal decomposition of the secondary acid generator is autocatalytic. The imaging medium may include a supersensitizer of the types previously described.
Application Ser. No. 08/944,284, filed Oct. 6, 1997 describes processes which closely resemble that of the aforementioned application Ser. No. 08/757,195 and which uses a secondary acid generator having a first site bearing a first leaving group and a second site bearing a second leaving group, the first leaving group being capable of protonation by the acid produced by the cationic electron acceptor, with expulsion of the first leaving group. However, in this application, the cation formed by expulsion of the first leaving group electrophilically adds to an unsaturated reagent bearing a proton at the site of addition and a proton-containing nucleophilic grouping at an adjacent site. After this electrophilic addition, the proton on the reagent is lost and the second leaving group of the secondary acid generator is displaced by the nucleophilic grouping on the reagent. As in the application Ser. No. 08/757,195, the second leaving group, in combination with a proton, forms the secondary acid.
The entire disclosures of the aforementioned patents and applications, all of which are assigned to the same assignee as the present application, are herein incorporated by reference.
It has now been found that the sensitivity of acid-generating media comprising a cationic electron acceptor, a sensitizing dye and a supersensitizer can be markedly increased by using as the supersensitizer a compound containing certain ionic groupings.